This invention relates to nuclear magnetic resonance (NMR) imaging methods. More specifically, the invention relates to improved NMR imaging methods which overcome undesirable effects of inherent static magnetic field inhomogeneity and minimize radio frequency (RF) power requirements.
The nuclear magnetic resonance phenomenon occurs in atomic nuclei having an odd number of protons and/or neutrons. Each such nucleus has a net magnetic moment such that when placed in a static homogeneous magnetic field, B.sub.o, a greater number of nuclei align with the B.sub.o field to create a net magnetization, M, in the direction of the field. Net magnetization M is the summation of the individul nuclear magnetic moments. Because a nuclear magnetic moment is the result of a nuclear spin, the terms "nuclear moment" and "nuclear spin" as used herein are synonymous.
Under the influence of the magnetic field B.sub.o, the nuclei (and hence the net magnetization M) precess or rotate about the axis of the field. The rate (frequency) at which the nuclei precess is dependent on the strength of the applied magnetic field and on the nuclei characteristics. The angular frequency of precession, .omega., is defined as the Larmor frequency and is given by the equation EQU .omega.=.gamma.B.sub.o ( 1)
in which .gamma. is the gyromagnetic ratio (constant for each type of nucleus) and B.sub.o is the strength of the applied static homogeneous magnetic field. The frequency at which the nuclei precess is thus primarily dependent on the strength of the magnetic field B.sub.o, and increased with increasing field strength.
A precessing nucleus is capable of absorbing electromagnetic energy. The frequency of the electromagnetic energy needed to induce resonance is the same as the precession frequency .omega.. During the application of the electromagnetic energy, typically a radio frequency pulse, the net magnetization M precesses further and further away from the z-axis (arbitrarily assumed to be the direction of the B.sub.o field), depending on the energy and duration of the RF pulse. A 90.degree. RF pulse causes the magnetization M to depart 90.degree. from the direction of the B.sub.o field into the x-y plane defined by the x- and y-axis, for example, of the Cartesian coordinate system. Similarly, a 180.degree. RF pulse causes the magnetization M to reverse direction by 180.degree. from its original direction (from the positive z-axis direction to negative z-axis direction, for example). Following the excitation of the nuclei with RF energy, the absorbed energy is re-radiated as an NMR signal as the nuclei return to equilibrium. The energy is emitted as radio waves and also transferred to surrounding molecules.
It is possible to distinguish NMR signals arising from different spatial positions in the sample based on their respective resonant frequencies. If one or more magnetic field gradients of sufficient strength to spread out the NMR signal spectrum are applied to the sample, each nuclear spin along the direction of the gradient experiences different magnetic field strength and hence resonates at a frequency different from that of other nuclear spins, as predicted by equation (1). Spatial positions of the NMR signals are determined by Fourier analysis and knowledge of the configuration of the applied magnetic field gradient.
The return of the nuclear spins to equilibrium following RF excitation is referred to as "relaxation". The relaxation process is characterized by two time constants, T.sub.1 and T.sub.2, both of which are measures of molecular level motion. The spatial distribution of T.sub.1 and T.sub.2 throughout the sample provides useful imaging parameters in addition to the density of nuclear moments or spins.
T.sub.1 is referred to as the "longitudinal" or "spin-lattice" NMR relaxation time and is a measure of the return of magnetization to equilibrium; i.e., the tendency of the nuclear spins to realign with the field B.sub.o after cessation of RF excitation. The rate of return to equilibrium is dependent on how fast energy can be transferred to surrounding material (lattice). T.sub.1 can vary from a few milliseconds in liquids to minutes or hours in solids. In biological tissue, the typical range is from about 30 milliseconds to 3 seconds.
T.sub.2, the transverse relaxation time or "spin-spin" relaxation time, is a measure of how long excited nuclear spins oscillate in phase. After a RF pulse, the nuclear spins are in phase and precess together. Each nuclear spin behaves like a magnet which generates a magnetic field that affects other spinning nuclei in its vicinity (spin-spin interaction). As each nuclear moment thus experiences slightly different magnetic fields, it precesses at a different rate and dephases with respect to the other spins, reducing the observed NMR signal. T.sub.2 can vary from a few microseconds in solids to seconds in liquids and is always less than or equal to T.sub.1. In biological tissue the range is about 5 milliseconds to 3 seconds.
If the static magnetic field B.sub.o itself has inherent inhomogeneities (as is often the case in practical magnets), these produce additional dephasing which hasten the decay of the NMR signal. This is because nuclear spins in different spatial positions are exposed to slightly different magnetic field values and hence resonate at slightly different frequencies. This new relaxation time, which includes the effects of magnet inhomogeneities, is designated T.sub.2 * (T.sub.2 star), where T.sub.2 *.ltoreq.T.sub.2.
Free induction decay (FID) and the spin-echo are among the methods by which the NMR signal may be observed.
In FID, the nuclear spins are irradiated with a RF pulse (90.degree., for example). Upon termination of the RF pulse, the spins produce an RF magnetic field as they precess. The NMR signal is observable as long as the nuclear spins precess in phase. The signal decays as the spins dephase and the decay curve is called the FID. If the B.sub.o static magnetic field is perfectly homogeneous, the decay curve is exponential with time constant T.sub.2. Otherwise, the decay is measured by T.sub.2 * which is apparatus dependent and not representative of the true T.sub.2 relaxation time of the sample. Under these circumstances, FID is not an acceptable method of measuring T.sub.2.
In the spin-echo method the nuclear spins are first subjected to a 90.degree. RF pulse, as in FID, and then to a 180.degree. RF pulse which creates the spin echo. Following the 90.degree. RF pulse, the nuclear spins precess in phase but quickly get out of phase due to inhomogeneities in the static magnetic field B.sub.o, as in FID. This loss of coherence can be reversed by the application of a 180.degree. RF pulse which reverses in the direction of diverging spins and results in a "spin echo". The initial curve of the spin echo signal is a mirror image of the original FID signal as the nuclear spins regain coherence. The second portion of the curve duplicates the original FID signal. The spin echo has a lower intensity due to irreversible losses attributable to the T.sub.2 relaxation process. The decay in the height of a series of such echoes can be used to calculate the T.sub.2.
The NMR imaging pulse sequences in accordance with the present invention overcome the T.sub.2 * effects caused by the inhomogeneities of the static magnetic field used in NMR imaging systems. Also, because the imaging gradients are turned off during the RF pulse, long RF pulse lengths can be used, thus reducing RF power requirements.